In general, in mobile communication systems, a plurality of cells adjacent to each other is formed, and for example, a mobile terminal in each of the cells such as a cellular phone performs wireless communication with a radio base station device installed in the center of the cell. Accordingly, the service area is expanded by installing a new radio base station device and forming a new cell. However, it is very expensive to install a radio base station device. For example, various costs such as costs for manufacturing or purchasing a radio base station device, costs for obtaining land on which the radio base station device is established, and costs for supplying operational power, are required to install the radio base station device.
To reduce these costs, in recent years, a method of individually installing a radio unit of a radio base station device as a separate body has been studied. In other words, it has been studied that a portion corresponding to the radio unit of the radio base station device is separated as a remote radio head (RRH) equipped base station device and the service area is expanded by installing a new RRH-equipped base station device instead of the radio base station device. Because the RRH-equipped base station device is smaller and consumes lower power than the radio base station device, it is possible to install the RRH-equipped base station device at a relatively low cost.
By using such an RRH-equipped base station device, a service area may be established along a narrow area such as a highway at a low cost. In other words, a service area that covers the entire narrow area can be effectively established, by connecting one base station device that is a master station and a plurality of RRH-equipped base station devices that is slave stations in cascade through an optical cable. Accordingly, each of the RRH-equipped base station devices connected in cascade performs wireless communication with a mobile terminal, and performs wired communication with the base station device, which is a master station, through an optical cable (see, for example, Japanese Laid-open Patent Publication No. 2000-349768).
A structure of connecting the RRH-equipped base station devices in cascade in this manner is also proposed, for example, in a common specification called a Common Public Radio Interface (CPRI) (see, for example, CPRI Specification V2. 1, “Common Public Radio Interface (CPRI); Interface Specification” Mar. 31, 2006). The RRH-equipped base station device proposed in the CPRI and the like also transfers data exchanged between the base station device and the other RRH-equipped base station device. For example, in the downlink from a base station device to an RRH-equipped base station device, each of the RRH-equipped base station devices provided between the base station device and the transmission destination RRH-equipped base station device transfers data to the adjacent RRH-equipped base station device from one to another. Eventually, the data is transferred to the transmission destination RRH-equipped base station device. Similarly, in the uplink from the RRH-equipped base station device to the base station device, each of the RRH-equipped base station devices provided between a transmission source RRH-equipped base station device and the base station device transfers data to the adjacent RRH-equipped base station device from one to another. Eventually, the data is transferred to the base station device.
In general, in the downlink communication in such a cascade structure, a data transmission source is always one master station (such as a base station device). However, in the uplink communication, the data transmission source may be a plurality of slave stations (such as RRH-equipped base station devices). Accordingly, in the uplink, each of the slave stations can only transmit a limited amount of data. In other words, in the uplink, each of the slave stations is not allowed to occupy the entire band to transmit data. Consequently, for example, a band available for transmitting data is equally assigned to each slave station (see, for example, Japanese Laid-open Patent Publication No. 2006-180279).
As described above, in the uplink in which one master station and a plurality of slave stations are connected in cascade, if one of the slave stations occupy the entire band, the other slave stations are not allowed to transmit data. Accordingly, a band is equally assigned to each of the slave stations, and the slave station transmits data by using the band assigned to the slave station. For example, if five slave stations are connected to one master station in cascade, and if the entire uplink band is 100 megabits per second, a band of 20 megabits per second is assigned to each of the slave stations. To transmit data to the master station, the slave station transmits data by using the band of 20 megabits per second assigned to the slave station. Accordingly, all the slave stations can transmit data equally.
However, when a band is equally assigned to the slave stations connected in cascade, if any one of the slave stations is not transmitting data, the band is not effectively used. In other words, in the above example, for example, if only one slave station is transmitting data to the master station, even if data can be transmitted in the band of 100 megabits per second, only the band of 20 megabits per second is used. Accordingly, the remaining band of 80 megabits per second is wasted.
Similarly, even if all the slave stations are transmitting data, if the amount of data transmitted from any one of the slave stations is small, the slave station may not use the entire assigned band. Accordingly, the band is wasted. If the amount of data to be transmitted to the master station varies among the slave stations, the slave station that transmits a large amount of data can only transmit data within the assigned band, even if unoccupied band is available in the entire line. As a result, the time in which all the slave stations complete data transmission is delayed, thereby lowering the throughput.